For upload an item on codecanyon or envato, you must create a documentation file. But how must it be? No need to search on internet. make it simple.
But generally, users on envato creates documentation file via Documenter. You can see so much examples of it on marketplace.
Here you can see an example of it: click here to see.
You can even make simple documentation.txt file. Dont worry. documentation side is the simplest one. Do focus on codes. Envato rejects most of projects 🙂
If you are rejected, you can upload your item to Codeograph. It is more simple and good alternative to envato.